Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper provides a covering for the foot that comfortably receives and securely positions the foot with respect to the sole structure. The sole structure is secured to a lower portion of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces, the sole structure may provide traction, control foot motions (e.g., by resisting pronation), and impart stability, for example. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of athletic activities.
The sole structure generally incorporates multiple layers that are conventionally referred to as a sockliner, a midsole, and an outsole. The sockliner is a thin, compressible member located within the upper and adjacent to a plantar (i.e., lower) surface of the foot to enhance footwear comfort. The midsole is conventionally secured to a lower surface of the upper and forms a middle layer of the sole structure that is primarily responsible for attenuating ground reaction forces. The outsole forms the ground-contacting element of the footwear and is usually fashioned from a durable, wear-resistant material that includes texturing to improve traction.
The conventional midsole is primarily formed from a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness and degree of ground reaction force attenuation may be altered to meet the specific demands of the wearer or of the activity for which the footwear is intended to be used.
In addition to polymer foam materials, conventional midsoles may include, for example, one or more fluid-filled chambers. In general, the fluid-filled chambers are formed from an elastomeric polymer material that is sealed and pressurized. The chambers are then encapsulated in the polymer foam of the midsole such that the combination of the chamber and the encapsulating polymer foam functions as the midsole of the sole structure. In some configurations, textile or foam tensile members may be located within the chamber or reinforcing structures may be bonded to an exterior or interior of the chamber to impart shape to the chamber.
Articles of athletic footwear are designed with a particular purpose in mind. Some articles of athletic footwear are designed to withstand jarring impact. Others are designed to withstand lateral impact. Some are designed to enhance stability. Others are designed to provide enhanced cushioning. The purpose for which a shoe will be used informs the design choices made by the designers.
Turning to FIGS. 3 and 4, prior art articles of footwear are shown. FIG. 3 shows an article of footwear 300 that is typical of a running shoe. When a runner wants to make a turn, he or she will plant a foot, which often creates a lateral force on the shoe. The midsole of the shoe will absorb some of the impact, but after some impact has been absorbed, the lateral force of the foot within the shoe 300 causes the shoe 300 to tip, as can be seen in FIG. 3. In addition, the lateral force absorption is noticeably lacking when the shoe 300 is examined on a slope, such as is shown in FIG. 4. In some cases, particularly in trail running, the terrain is not flat, instead including many irregular surface patterns. When the runner plants a foot on such irregular terrain, the midsole can only deform slightly and does not absorb sufficient lateral forces. This creates strain on the runner's foot and ankle.